Overview of Existing Lunar Base Structural Concepts

Various structural concepts for lunar bases have been proposed and considered during the past three decades, each balancing the multitude of lunar constraints and anticipated base functions with a different distribution of weighting factors. This paper provides a brief review of the study of lunar base structures as well as an overview of current ideas. Lunar environmental characteristics are highlighted, and questions of structural functionality are categorized. In an appendix to the overview report, building systems proposed for lunar applications are categorized according to applications, application requirements, types of structures, material considerations, structures technology drivers, and requirement definitions. Another appendix presents a short list of the most important references related to these issues, as ranked by the task committee. This document contains a brief overview rather than an exhaustive review of concepts proposed for lunar outposts. The original documents, including those cited as references, are not replaced by this paper. However, this review is intended to be reasonably complete. There has been no conscious intention to ignore any work in this area of extensive ongoing activity.     

Structural Design of a Lunar Habitat

A lunar base is an essential part of all the new space exploration programs because the Moon is the most logical first destination in space. Its hazardous environment will pose challenges for all engineering disciplines involved. A structural engineer’s approach is outlined in this paper, discussing possible materials and structural concepts for second-generation construction on the Moon. Several different concepts are evaluated and the most reasonable is chosen for a detailed design. During the design process, different solutions—for example, for the connections—were found. Although lunar construction is difficult, the proposed design offers a relatively simple structural frame for erection. A habitat on the Moon can be built with a reasonable factor of safety and existing technology. Even so, we recognize the very significant difficulties that await our return to the Moon.     

Technical Issues for Lunar Base Structures

The establishment of a permanent human presence on other planets will require establishing permanent infrastructure in new environments. Civil engineers select, define, and implement solutions to infrastructure design problems in unique environmental contexts. Wind and seismic loading are two examples of constraints long familiar to terrestrial civil engineering. Designing structures for lunar exploration, development and eventual settlement will make use of the same design processes already practiced by the civil engineering profession. However, the extensive experience base resulting from centuries of terrestrial work does not adequately prepare civil engineers for the unprecedented constraints and environmental conditions that are encountered in space. The limited knowledge we already have about the Moon (mostly from the Apollo program) is a place to start. By assimilating and working with this knowledge, those pursuing the design of lunar base structures can begin to produce realistic and valid design solutions. The paper presents technical, operations, and programmatic issues that the writers consider fundamental to understanding the facts of life in this promising new design arena.     

Geometric Modeling of Inflatable Structures for Lunar Base

A modular inflatable structure consisting of thin, composite membranes is presented for use in a lunar base. Results from a linear elastic analysis of the structure indicate that it is feasible in the lunar environment. Further analysis requires solving nonlinear equations and accurately specifying the geometries of the structural members. A computerized geometric modeling technique, using bicubic Bezier surfaces to generate the geometries of the inflatable structure, was conducted. Simulated results are used to create three‐dimensional wire frames and solid renderings of the individual components of the inflatable structure. The component geometries are connected into modules, which are then assembled based upon the desired architecture of the structure.     

Technical Requirements for Lunar Structures

The moon has recently regained the interest of many of the world’s space agencies. Lunar missions are the first steps in expanding manned and unmanned exploration inside our solar system. The moon represents various options; it can be used as a laboratory in low gravity, it is the closest and most accessible planetary object from the Earth, and it possesses many resources that humans could potentially exploit. This paper has two objectives: to review the current status of the knowledge of lunar environmental requirements for future lunar structures, and to attempt to classify different future lunar structures based on the current knowledge of the subject. The paper divides lunar development into three phases. The first phase is building shelters for equipment only; in the second phase, small temporary habitats will be built, and finally in the third phase, habitable lunar bases will be built with observatories, laboratories, or production plants. Initially, the main aspects of the lunar environment that will cause concerns will be lunar dust and meteoroids, and later will include effects due to the vacuum environment, lunar gravity, radiation, a rapid change of temperature, and the length of the lunar day. This paper presents a classification of technical requirements based on the current knowledge of these factors, and their importance in each of the phases of construction. It gives recommendations for future research in relation to the development of conceptual plans for lunar structures, and for the evolution of a lunar construction code to direct these structural designs. Some examples are presented along with the current status of the bibliography of the subject.     

Design and Construction Considerations for Lunar Outpost

Engineers utilize various codes in the process of design, whether structural, mechanical, or otherwise. Reliance on a code for design is based on the knowledge that a tremendous amount of time and effort was spent by experienced engineers to codify theories and good practice in a particular design discipline. Good practice in structural design implies cognizance of materials, structural behavior, environmental loadings, assumptions made in analysis and behavior, and the uncertainties inherent in all of these. The American Institute of Steel Construction's (AISC) Manual of Steel Construction is such a codification for the design and construction of steel structures. It includes information, some tabular and the rest in the form of specifications and commentaries, necessary to design and provide for the safe erection of steel‐framed structures. The design equations are generally semiempirical, that is, they are based on a mix of theoretical analysis, experimental data, and factors of safety. Each of these components has associated implicit assumptions. Some of these assumptions were explored to understand how and if the Earth‐based design code could be used for the design of a lunar outpost. Topics discussed come from the AISC Code of Standard Practice and the commentaries, and issues such as scaling of loads and strength in the 1∕6 g lunar environment, thermal cycling effects and fatigue, stiffening and buckling are briefly discussed. Important topics for further detailed study include: (1) The relationships between severe lunar temperature cycles and fatigue; (2) very low temperature effects and the possibility of brittle fractures; (3) outgassing for exposed steels and other effects of high vacuum on steel∕alloys; (4) factors of safety originally developed to account for uncertainties in the Earth design∕construction process undoubtedly need adjustment for the lunar environment; (5) dead loads∕live loads under lunar gravity; (6) buckling∕stiffening and bracing requirements for lunar structures that will be internally pressurized; and (7) consideration of new failure modes such as high‐velocity micrometeorite impacts.     

Considerations for Design Criteria for Lunar Structures

The development of design criteria for lunar structures must begin soon in order to establish adequate criteria. Some of the items that need consideration in such criteria are discussed. The categorization of the structures will provide designers with information on the purpose and level of complexity of the structure. Various construction materials and structure types that will be critical for the design of lunar structures, are considered. The environment of the moon and its possible effects on structures are presented and lead to the development of a few load cases that need to be considered in design. A probabilistic format for the criteria and design lifetimes are also discussed.     

Concepts for Lunar Outpost Development

This paper summarizes the results of a qualitative investigation to identify concepts for design and construction of near‐term lunar facilities. Accomplishing such construction will require an adaptation or transfer of current terrestrial technology and methods. Discussions on modularization, geosynthetic materials, aluminum materials, static load analysis, and dynamic load analysis provide illustrative examples of how terrestrial technologies can be adapted to lunar applications. These discussions provide support for the development of a phased lunar construction strategy. The initial stage of construction is characterized by small self‐supporting accomodation and laboratory modules. The assembly facility stage is characterized by the construction of a large pressurized module‐assembly facility. The module production stage is characterized by the fitting together of terrestrial or low earth‐orbit subassemblies into completed modules within the module assembly facility. The completed modules are also tested and moved to their final location in this stage. The lunar materials stage is characterized by the construction of facilities with maximum use of lunar materials.     

Lunar‐Base Construction Equipment and Methods Evaluation

A process for evaluating lunar‐base construction equipment and methods concepts is presented. The process is driven by the need for more quantitative, systematic, and logical methods for assessing further research and development requirements in an area where uncertainties are high, dependence upon terrestrial heuristics is questionable, and quantitative methods are seldom applied. Decision theory concepts are used in determining the value of accurate information and the process is structured as a construction‐equipment‐and‐methods selection methodology. Total construction‐related, earth‐launch mass is the measure of merit chosen for mathematical modeling purposes. The work is based upon the scope of the lunar base as described in the National Aeronautics and Space Administration's Office of Exploration's “Exploration Studies Technical Report, FY 1989 Status.” Nine sets of conceptually designed construction equipment are selected as alternative concepts. It is concluded that the evaluation process is well suited for assisting in the establishment of research agendas in an approach that is first broad, with a low level of detail, followed by more‐detailed investigations into areas that are identified as critical due to high degrees of uncertainty and sensitivity.     

Mechanical Equipment Requirements for Inflatable Lunar Structures

Inflatable structures have been proposed by a number of authors. Several structural forms have been conceptually designed, including spherical, pillow‐shaped, semicylindrical, and domed saucer. Regardless of structural form, all inflatables require mechanical equipment to initiate and maintain inflation. This paper identifies the mechanical equipment and operations required to support an inflatable structure. A previously proposed semicylindrical structure is selected for this study, but the principal results are applicable to all inflatable structures. The results indicate that air for inflatable structures should be transported to the moon in a liquid (cryogenic) state. The liquefied air can be evaporated and heated to the proper temperature using solar energy and a conventional pumping system. Removing the air from the facility is an entirely different problem and requires different equipment. There are two alternatives: (1) Discharge the air to the moon; and (2) reclaim the air for reuse. The first alternative is not likely to be cost‐effective and might well be scientifically unacceptable. The second alternative presents numerous technical problems but appears technically feasible.     

Lunar Astronomical Observatories: Design Studies

The best location in the inner solar system for the grand observatories of the 21st century may be the Moon. A multidisciplinary team including university students and faculty in engineering, astronomy, physics, and geology, and engineers from industry is investigating the Moon as a site for astronomical observatories and is doing conceptual and preliminary designs for these future observatories. Studies encompass lunar facilities for radio astronomy and astronomy at optical, ultraviolet, and infrared wavelengths of the electromagnetic spectrum. Although there are significant engineering challenges in design and construction on the Moon, the rewards for astronomy can be great, such as detection and study of Earth‐like planets orbiting nearby stars, and the task for engineers promises to stimulate advances in analysis and design, materials and structures, automation and robotics, foundations, and controls. Fabricating structures in the reduced‐gravity environment of the Moon will be easier than in the zero‐gravity environment of Earth orbit, as Apollo and space‐shuttle missions have revealed. Construction of observatories on the Moon can be adapted from techniques developed on the Earth, with the advantage that the Moon's weaker gravitational pull makes it possible to build larger devices than are practical on Earth.     

Geotechnical Investigation Strategies for Lunar Base

Geotechnical characterization of potential lunar sites will be a critical part of the planning and design process. The strategies used to conduct a geotechnical investigation will be dictated by the specific needs of the lunar base, the unique environment of the lunar surface, and general character of the lunar soils and rocks. This paper outlines some of the types of geotechnical information that will be important and identifies some of the more promising strategies which might be used to obtain such information in the lunar environment. Some of the most important geotechnical information for planning and site development will be related to construction in the lunar soil. In addition to construction concerns, geotechnical data for foundation design (or verification of predesigned foundations) will be needed. The geotechnical site‐characterization work should include geophysical techniques, supplemented by conventional mechanical boring and testing only to the degree necessary to correlate geophysical measurements with conventional soil properties and to investigate anomalies. Equipment used for geotechnical site characterization will also serve for mineralogical exploration. Several techniques for geotechnical investigation that may provide very useful information in an expedient manner are described. Geophysical methods include seismic and electromagnetic methods, including seismic surveys that utilize surface waves. Electromagnetic methods such as ground‐penetrating radar are fast, efficient methods for mapping the subsurface, although these techniques do not measure soil characteristics that can readily be correlated with engineering properties. Seismic methods provide information that may correlate with soil strength, compressibility, and excavatability. In‐situ physical testing will likely include penetration testing for direct physical measurement of lunar soil behavior.     

Structural Design of Lunar Radio Telescope Using Interactive CAD

Some astronomers are considering the moon as an attractive location within the inner solar system for a variety of astronomical observatories, some of which could be operational early in the 21st century. This paper describes the computer‐aided structural design of a 122‐m diameter, fully steerable, parabolic radio telescope to be located on the moon. The loads acting on such a reflector differ substantially from those acting on a reflector that must operate in earth's environment. The moon has excellent advantages as a location for such an instrument. The absence of atmosphere completely eliminates the wind, snow, and ice loads. The gravity field is only one‐sixth that of earth's. The thermal changes from night to day are severe, but structural problems can be avoided by using a thermally stable composite material. Structural elements for the reflector dish have been analyzed and designed for static loads with a specially written interactive graphical computer program. The resulting structure has a mass nearly 40 times less than its earth's counterpart made of steel. The evaluation of the results of the design studies supports the possibility of building a large‐aperture parabolic radio telescope on the moon.     

Indigenous Resource Utilization in Design of Advanced Lunar Facility

The most important consideration in the establishment and support of a permanently manned lunar base will be resource utilization. Seven potential lunar construction materials were analyzed with respect to their physical properties, processes, energy requirements, and resource efficiency. Reviewing the advantages and disadvantages of each material led to the selection of basalt as the primary construction material for initial use on a lunar base. The team conceptualized a construction system that combines lunar regolith sintering and casting to make pressurized structures. The design uses a machine that simultaneously excavates and sinters the lunar regolith to create a cylindrical hole. The hole is then enclosed with cast basalt slabs, allowing the volume to be pressurized for use as a living or work environment. Cylinder depths up to 4–6 m in the lunar mare and 10–12 m in the lunar highlands can be achieved. Advantages identified in the construction system include maximum resource utilization, relatively large habitable volumes, interior flexibility, and minimal construction equipment needs. The conclusions of this study indicate that there is significant potential for the use of basalt as a low‐cost alternative to Earth‐based materials. It remains to be determined, during lunar base phasing, whether this construction method should be implemented.     

Bioinspired Computational Framework for Enhancing Creativity, Optimality, and Robustness in Design

This paper presents results of transdisciplinary research on the development of a bioinspired computational framework for engineering design. This framework is intended to support design by addressing three critical design objectives, including novelty, optimality, and robustness. It provides several computational models and methods, which are inspired by fundamental processes occurring in nature, and discusses their potential for enhancing design. They include models and methods for evolutionary, developmental, and coevolutionary design. Their use is illustrated with examples from the area of steel structural design ranging from a simple cantilever beam design problem to a much more complex problem of designing wind bracings in tall buildings. The paper also shows how several methods and models can be integrated and form a coherent bioinspired computational framework for engineering design.     

MALEO: Modular Assembly in Low Earth Orbit: Alternate Strategy for Lunar Base Development

Modular assembly in low Earth orbit (MALEO) is a new strategy for building an initial operational‐capability lunar habitation base, the main purpose of which is to safely initiate and sustain early lunar base buildup operations. In this strategy the lunar base components are brought up to low Earth orbit (LEO) by the Space Transportation System (STS), and assembled there to form the complete lunar base. Specially designed propulsion systems are then used to transport the MALEO lunar base, complete and intact, all the way to the moon. Upon touchdown on the lunar surface, the MALEO lunar habitation base is operational. The strategy is unlike conventional concepts, which have suggested that the components of the lunar base be launched separately from the Earth and landed one at a time on the moon, where they are assembled by robots and astronauts in extravehicular activity (EVA). The architectural drivers for the MALEO concept are, first, the need to provide an assured safe haven and comfortable working environment for the astronaut crew as safely and as quickly as possible, with the minimum initially risky EVA, and secondly, the maximum exploitation of the evolutionary benefits derived from the assembly and operation of space station Freedom (SSF‐1). Commonality and inheritability from the space station assembly experience is expected to have an advantageous impact on both the space station program as well as the MALEO lunar base.     

Concept Evaluation Methodology for Extraterrestrial Habitats

The paper examines and compares structural concepts considered for use as habitats for lunar and Martian outposts. An evaluation methodology that allows numeric rating of concepts was previously developed and is upgraded herein. The methodology defines a number of important characteristics on which concepts are to be judged. In addition, weighting factors are assigned for the various characteristics considered in the evaluation system. These factors are presented as variables that depend on mission goals and timing aspects. An example evaluation is made for a specific scenario utilizing the developed methodology. The overall purpose of this work is not to provide an absolute rating, but rather to identify strengths and weaknesses of concepts. This approach should be invaluable in the development and selection of structural concepts for extraterrestrial habitats.     

Augmented Lagrangian Genetic Algorithm for Structural Optimization

This paper presents a robust hybrid genetic algorithm for optimization of space structures using the augmented Lagrangian method. An attractive characteristic of genetic algorithm is that there is no line search and the problem of computation of derivatives of the objective function and constraints is avoided. This feature of genetic algorithms is maintained in the hybrid genetic algorithm presented in this paper. Compared with the penalty function‐based genetic algorithm, only a few additional simple function evaluations are needed in the new algorithm. Furthermore, the trial and error approach for the starting penalty function coefficient and the process of arbitrary adjustments are avoided. There is no need to perform extensive numerical experiments to find a suitable value for the penalty function coefficient for each type or class of optimization problem. The algorithm is general and can be applied to a broad class of optimization problems.     

Processing of Lunar Materials

A variety of products made from lunar resources will be required for a lunar outpost. These products might be made by adapting existing processing techniques to the lunar environment, or by developing new techniques unique to the moon. In either case, processing techniques used on the moon will have to have a firm basis in basic principles of materials science and engineering, which can be used to understand the relationships between composition, processing, and properties of lunar‐derived materials. These principles can also be used to optimize the properties of a product, once a more detailed knowledge of the lunar regolith is obtained. Using three types of ceramics (monolithic glasses, glass fibers, and glass‐ceramics) produced from lunar simulants, we show that the application of materials science and engineering principles is useful in understanding and optimizing the mechanical properties of ceramics on the moon. We also demonstrate that changes in composition and/or processing can have a significant effect on the strength of these materials.     

Analysis of a Lunar Base Structure Using the Discrete-Element Method

A lunar base structure must provide protection against various hazards such as bombardment by meteorites, radiation, or extreme changes in temperature. A possible structural solution was proposed in the literature. The lunar base, planned to be built in a long, narrow valley with solid rock walls, would consist of three main elements: a masonry vault, supported by the rock walls of the valley; a regolith layer a few meters thick on top of it to dissipate radiation and the kinetic energy of impacting meteorites, and to balance temperature; and inflatable units under the arch to serve as a living area for humans. The objective was to check the feasibility of this idea from a structural mechanics point of view. A two-dimensional discrete-element model of the vault-regolith system was constructed, and the behavior of the structure under its own weight was analyzed. Initial simulations on the effect of meteorite impacts were conducted, but a significantly improved model would be required to continue this analysis. This note summarizes the results to date.